This invention relates to gages for measuring inside and outside diameters of tubular products and tubular portions of products, the outside diameters of solid goods, or the inside diameter of circular apertures, the surfaces of which may be smooth or threaded and straight or tapered. In the particular embodiment disclosed herein, this invention concerns a gage which is particularly adapted for measuring the pitch diameter of internally or externally, straight or tapered pipe threads at any desired distance from the end of the pipe.
Various components used in the oil and gas production industry are provided with threads at their ends to join one to another. These components include line pipe, casing, tubing, couplings, rotary joints, and all other connections. The threads may be straight or tapered, and the thread form may be of the round or buttress type, or any other thread form used in the petroleum industry, including premium connections.
For use in the field, it is important for proper performance and extended service life of the components that the joints between the various components be strong, durable, and tight. This requires that the threads used to form these joints be fabricated with precision. This is particularly important for tapered thread.
To obtain high precision, it is desirable that thread sizes and configurations for threaded components used in oil and gas production as well as other applications be standardized for the particular industry so that quality control can be uniformly maintained throughout the industry. In the oil and gas production industry, the American Petroleum Institute (API) has adopted a specification for threading, gaging and thread inspection of components used in the petroleum production industry. This specification sets forth standard dimensions for pitch diameters for the various types and sizes of tubing and pipe threads corresponding to a range of pipe and tubing diameters. The specification also sets forth the distance from the end of the threaded member to the point on the tapered thread where the standard pitch diameter is to be measured.
In addition to specifying the standard pitch diameter dimensions, the API specification also sets forth a procedure for checking the accuracy of the actual pitch diameter of the threaded product compared to the standard pitch diameter. This procedure, which is universally accepted and used by the petroleum production industry, utilizes a series of ring and plug gages to check the accuracy of the pitch diameter against the API standard pitch diameter for the particular thread configuration and pipe size. This series of gages includes a plug and mating ring for each thread configuration covered by the API specification. These thread configurations vary by pipe diameter size, taper, pitch and type of thread, e.g., round or buttress. The total number of thread configurations covered by the API's specification amounts to some 60 configurations, and a manufacturer desiring to fabricate all of these thread configurations therefore needs a like number of plug and mating ring pairs. This number, of course, does not include the many non-API thread forms now in use.
Further, the API specification also requires the manufacturer to have two different sets of plug and mating ring pairs, a set of reference master gages and a set of working gages. The working gages are used for actually gaging the product threads, while the reference master gages are used mainly to periodically check the accuracy of the working gages and only rarely to check the product threads. Because the rings and plugs are made of relatively expensive materials and require substantial machining to very precise tolerances, a complete set of plugs and mating rings costs hundreds of thousands of dollars. Also, the threads of the rings and plugs eventually wear out from repeated use, necessitating replacement.
Ring and plug gages are used to check the accuracy of the pitch diameter at a particular point from the end of a pipe as specified by the API specification. Ring and plug gages do not, however, directly measure the pitch diameter of a product thread at the specified distance from the pipe end. Rather, the ring or plug is threaded onto the product threads until hand tight engagement is achieved. Then the so-called standoff of the ring or plug from the pipe is checked against a standoff defined by the API specification. For a plug, which is used to check internal threads, the standoff is the distance from a step portion of the plug to the nose of the product. For a ring, the standoff is measured from the nose of the product to the rear end of the ring. Both standoffs are checked against a corresponding distance for the reference plug and ring pair in hand tight engagement, and both must fall within a tolerance as compared to the reference plug and ring standoffs as established by the API. Thus this gaging system only provides a comparison between actual standoff to a reference standoff.
As a result, the ring and plug gaging system established by the API possesses certain inherent shortcomings. Rings and plugs do not directly measure pitch diameter. Nor do they indicate whether the thread is out of round, i.e., ovality. Further, ring and plug gages are capable of measuring standoff to an accuracy of only about 1/64th of an inch. Finally, the position attained by hand tight engagement of the ring or plug may be affected by any of the following: damaged or distorted threads, improper thread form, incorrectly machined thread elements, ovality, cleanliness, or excessive lubrication.
Others have attempted to provide a pitch diameter gage which eliminates the need for a set of ring and plug gages. For example, such a gage is shown in U.S. Pat. No. 4,189,843 to Baldwin. However, this gage does not provide any precise quantitative measure of the deviation in pitch diameter from the standard. Nor does it specifically measure the pitch diameter at a selected axial distance from the end of the pipe, specifically the pitch diameter at the distance from the end of the pipe as specified by the API specification.